I'm here to share some information about the error you're getting in QuickBooks Online, @graham-hogg.
This has been reported as an ongoing issue with the investigation number INV-65271. Our Product Engineers are diligently working to get it fixed as soon as possible.
While we don't have a firm timeline for when this investigation will be resolved, I highly suggest contacting our Phone Support Team. I know you already contacted them but they're the ones to give updates about the investigation.
For now, you may import your bank transactions manually through CSV Import. You can use this article for a reference: Format CSV files in Excel to get bank transactions into QuickBooks.
For duplicate transactions, there are simple steps you can follow to resolve the problem. I recommend using the Exclude feature to remove duplicates from your banking. For more information, as well as how to exclude a transaction in the QuickBooks, I recommend the following article: Exclude a bank transaction you downloaded into QuickBooks Online.
Once you're all set, you can now continue categorise and reconcile your bank transactions. For more information, feel free to check out these articles:
Thank you for your patience while we work for a fix. Please post a reply below if you have other concerns. I'll be more than happy to assist you again. Have a great day and take care always, @graham-hogg.
